What's in Brian's Brain?

The Small World of the Brain

The late Valentino Braitenberg took a distinctively quantitative approach to the great puzzles of neuroscience. With his colleagues at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany, he spent more than two decades counting and measuring the cells of the mammalian cerebral cortex. The facts and figures he gathered did not solve all the mysteries, but any proposed answers will have to take his data into account.

A neuron in the human cortex (specifically, a pyramidal cell) has a frizzy nimbus of dendrites that extends throughout a volume of roughly 1 cubic millimeter. But the cell does not fill this entire volume; on the contrary, it shares the space with 100,000 other cells in a densely woven thatch of dendrites and axons. The combined length of all the dendrites in this volume is about 450 meters; the total length of the axons is even greater, more than 4 kilometers. In this tangled skein of nerve fibers, you might imagine that every cell would cross wires with every other cell. But Braitenberg found that most of the cells make no contact with one another. Choosing any two neurons that have fibers extending into the same cubic millimeter, the probability that they share a synapse is only 2 percent.

However, this sparse connectivity does not mean that the cortex breaks down into isolated clusters of cells. A signal from any neuron can reach all the other neurons in the entire cortex—all 20 billion of them—after passing through no more than two or three synapses. The cortex is a “small world” network, like social networks in which everyone is connected by chains of friends of friends.

On this evidence Braitenberg argued that the cortex is “a device for the most widespread diffusion and most intimate mixing of signals.” He suggested that this architecture is just what would be expected in an associative memory. Incoming signals spread rapidly throughout the cortex, reaching nearly all the neurons. Each possible combination of inputs evokes a different response: One set of cells is activated when we encounter quacking and waddling creatures, another set recognizes the barking and tail-wagging ones.

Questions remain about how such a system would reliably distinguish a boundless spectrum of possible patterns. At one extreme, every concept is associated with a single neuron; this is the notion of the “grandmother cell,” which lights up when granny enters the room. Cell assemblies might be seen as a broader version of the same idea, with overlapping populations of cells representing percepts and concepts. Braitenberg favored an even more diffuse scheme, in which concepts are embodied in the global state of the entire network.

Not everyone goes along with this view of the cortex as a large, undifferentiated memory organ. Indeed, there is much compelling evidence that regions have special functions, such as vision and speech. Braitenberg believed these opposing theories of the cortex could be reconciled.